The invention relates to a flexible skin-rib structure consisting of fiber composite material for use in flow profiles with variable camber, especially airfoils or their components, in which a suction-side skin with suction-side rib sections and a pressure-side skin with pressure-side sections are connected.
Modern airfoils are normally optimized for a limited range of applications. However, since a considerable portion of the take-off weight of an aircraft consists of the fuel that it carries, the weight of an aircraft decreases by up to 30% during flight. The lift/drag ratio (ratio of lift to drag) of the aircraft also deteriorates as a result of this increasing distance from the design point. With the aid of the variable camber of the trailing edge of the airfoil, for example, an adaptation can be made to the changed flight conditions. This means greater flexibility in application and more potential for additional developments later.
U.S. Pat. No. 3,109,613 teaches an internally adjustable, flexible skin-rib structure in which the pressure side and suction side skins are connected by rigid ribs, with the ribs being connected to the skins by hinge-like joints (piano hinges) to maintain flexibility.
A disadvantage of this design is that with camber, i.e. relative shifting of the skins with respect to one another, no bending moments can be introduced, leading to critical stresses in the vicinity of the skin-rib connection. Consequently this area is subjected to a double load: by the connection and by the bending moments. In addition, this design is difficult to create when fiber composite material is used to reduce weight, since a joint must be integrated into the skin-rib connection.
The goal of the present invention is to improve on the flexible skin-rib structure so that it is sufficiently rigid to handle aerodynamic stresses but remains capable of being deformed by small forces. It must also be economical and capable of being manufactured with lightweight construction. Another goal consists in relieving the stress on the connecting areas of the skins and ribs with a camber by reducing the stresses.
To achieve this goal, a skin-rib structure according to the invention of the type described above is characterized by the fact that the respective suction side and pressure side rib sections are connected with one another by joints.
In the skin-rib structure according to the invention, construction using fiber composite material is possible, guaranteeing extremely low weight; this avoids a situation in which the amount of fuel saved by adapting the airfoil profile is made up again by the additional weight of the material. In addition, the bending stresses that occur during camber are concentrated in the joint area inside the ribs, so that the skin-rib connecting area has the stress relieved and the maximum admissible material expansion values are not exceeded.
In one preferred embodiment, the suction-side rib sections are connected by suction-side joints with central rib sections, which in turn are connected by pressure-side joints with the pressure-side rib sections. In this form, the entire rib that joins the skins thus comprises a sequence composed of a suction-side rib section, a suction-side joint, a central rib section, a pressure-side joint, and a pressure-side rib section.
The fact that the rib sections and joints are arranged in a line offers several possibilities for aligning and attaching the joints between the rib sections. As a result of the movement transmitted to the joints during camber, the joints are preferably flectors. In one advantageous embodiment, the joints, like the rib sections, consist of fiber composite material. In this design, all of the rib sections and joints can be manufactured simultaneously when the ribs are made. In addition, a single component results that can be connected with the skins like a conventional rib without joints when the airfoil is constructed, thus saving on additional labor costs.
Preferably, the joints are produced by the suction side and pressure side rib sections tapering conically, then forming a joint where the thickness remains constant, and thickening conically again over a suction-side or pressure-side cone of the central rib section. In this manner, the joints are produced in the element itself, with the number of layers of fiber composite material being successfully reduced. The nature and form of this reduction of the layers can regulate both the shape of the conically-designed sections as well as the length and position of the rib sections and the length and position of the joints. The nature of the conical sections of the skin-side rib sections determines the distance of the joints from the skins. The stresses in the joints vary with the distance of the joints from the skins. However, the joint exerts the greatest influence on the stresses in the joint area itself by virtue of its length and thickness, with the stresses increasing with thickness. The joint length determines the area in which expansion is to take place. The shorter the length, the higher the stresses in the joint. The increased stiffness also increases the forces required for camber. Since the structure must be deformable by a limited expenditure of force while remaining sufficiently stiff to be able to accept aerodynamic loads, the skin-rib structure can be adapted to the various requirements by varying the joint thickness and joint length. These requirements can differ in an airfoil in the span direction, so that the joint length and joint thickness can also vary in this direction.
Since the ribs in the skin-rib structure according to the invention perform several tasks, it is advantageous for the alignment of the fibers in the fiber composite material to be adapted to these requirements, and there is a layered structure of the fibers that is adjusted to these requirements. Especially preferred is an embodiment in which the fibers in the joints run at angles of .+-.45.degree. and 90.degree. to the span direction, which is equivalent to saying that the 90.degree.-direction runs in the rib-height direction. As a result of the layers that run in the .+-.45.degree.-direction, the ribs can best accept the thrust that develops. Since the ribs in the structure according to the invention also are subjected to bending stress, the additional fiber layers preferably run in the rib-height direction to accept the bending stress.
With the preferred embodiment described above, the thrust and bending stresses in the joints can best be accepted and withstood. Since both the rib sections on the skin side and the central rib sections undergo a gradual thickening over the conical sections, the required stiffness of the skin-rib structure can be provided in the rib sections in addition to the fiber layers applied to the joint layers.
Advantageously, the entire ribs thus consist of two skin-side rib sections each and central rib sections that are connected by joints. All of these components consist of layers of fiber composite material. The only differences are in the number of layers and the orientation of the fibers. Thus, the joints consist of fewer layers than the rib sections and in part show a different orientation. In the rib sections, additional layers are applied to the already existing layers so that the fiber layers that form the joints also form the cores of the rib sections. The thickening takes place in stages over the conical sections.
The nature of the fiber composite material can remain constant over all of the applied layers, for example carbon-fiber-reinforced plastic, advantageously, however, the material in the different layers varies. Thus the 90.degree. layers are preferable made of glass-fiber-reinforced plastic while the .+-.45.degree. layers are made of carbon-fiber-reinforced plastic. The layers of the fiber composite material that are applied in the rib sections preferably consist of carbon-fiber-reinforced plastic.
The ribs that comprise the rib sections and the joints to form the skin-rib structure are connected with the respective skins by the pressure-side and suction-side rib sections.
This connection can be of any form but advantageously the pressure-side and suction-side rib sections are rigidly connected with the pressure-side and suction-side skins. Preferably the pressure-side and suction-side rib sections are permanently attached to the respective skins. Especially preferable, adhesive strips are formed on the pressure-side and suction-side rib sections, parallel to the skins. The strips are glued to the skins so that a rigid connection is guaranteed on both sides between the skin and the skin-side rib sections. The adhesive strips are formed when the layers of the fiber composite material that form the skin-side rib sections are bent at the angle between the skin and the rib during manufacturing. Thus the adhesive strips, skin-side rib sections, joints, and central rib sections form a unit and can be installed as a unit when the skin-rib structure is assembled. The hollow space formed by the adhesive strips and the skins is filled with the so-called gore, which is preferably made of carbon-fiber-reinforced plastic, guaranteeing a continuous connection between the skin and the rib.
The ribs can be connected to the skins, as described above, basically by an adhesive connection, but under certain conditions, for example with a particular load on the connection, it is advantageous for the skin-side rib sections to be riveted to the skins. At high peeling stresses, rivets prevent the adhesive layer from peeling away and causing the adhesive joint to fail.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.